Activated carbon products and their manufacture

ABSTRACT

An activated carbon product such as a charcoal cloth or felt has, in addition to any activating material, a metal uniformly dispersed therein. The metal may be catalytic or bactericidal, and a particularly preferred product of the invention contains silver and is suitable for use as a surgical dressing. Such products can be produced by a conventional activation procedure using a mixture of halides, even if the further metal halide is substantially insoluble in the impregnation solution used for activation, by incorporating in the solution ammonia and a sequestering agent such as citric acid.

This is a division of application Ser. No. 531,936, filed Sept. 13,1983, now U.S. Pat. No. 4,529,623.

This invention relates to activated carbon products, in particularactivated charcoal cloths and felts, and to methods for theirmanufacture. The products of this invention can be of, for example,bactericidal utility.

Various methods for producing activated charcoal cloths are known. Forexample, GB-A-1455531 discloses impregnating cellulose fibres with areactive phosphorus compound and heating the impregnated fibres undercertain conditions. The Kirk-Othmer Encyclopaedia of ChemicalTechnology, 16, 3rd Ed., 136 discloses the preparation of novoloid-basedactivated carbon in a one-step process, combining both carbonisation andactivation, in an oxygen-free atmosphere containing steam and/or CO₂, atabout 900° C. The products are said to have uniform pore size.

GB-A-1301101 discloses a particularly useful, and commercially used,process for preparing activated carbon products in fibrous form. Rayon,for example, is impregnated with a solution of inorganic halides, e.g. amixture of ammonium, zinc and hexahydrated aluminium chlorides. Theimpregnation is followed by a controlled heating stage.

The utility of a carbonised fabric in surgical dressings has beenappreciated for over 50 years. GB-A-386067 discloses surgical dressingscomprising woven or entangled carbonised fibres. Such dressings are alsodisclosed as supports for therapeutic or antiseptic materials and it isstated that "the dressings will hold in considerable quantities iodine,formol, lime, oxygen, bacillary toxins, and the like". The use of, say,iodine in the dressings disclosed in GB-A-386067 appears to be aconsequence of the adsorptive characteristics of charcoal cloths.Charcoal cloth is an excellent adsorbent for organic water-contaminantssuch as phenol, organic acids and insecticides. Charcoal cloth can alsobe used, e.g. in gas masks, to remove undesirable gases from the air.

EP-A-0053936 discloses surgical dressings comprising activated charcoal,preferably as activated charcoal cloth, impregnated with ananti-microbial agent, iodine being preferred. A characteristic of thisdisclosure is that no more than 20%, and preferably about 5%, of theadsorptive sites of the activated charcoal are saturated with theanti-microbial agent. Such a product probably contains less, say, iodinethan an impregnated product as disclosed in GB-A-386067, but stillsuffers from the disadvantage that iodine is easily removed from thecloth in the presence of aqueous media. It is generally consideredundesirable that free iodine should be allowed to come into contact witha wound, and yet this can be a problem associated with the use ofsurgical dressings, containing iodine, as disclosed in EP-A-0053936.

Charcoal cloth has considerable utility of its own as a wound dressing.It can adsorb unpleasant odours of the type which often emanate frominfected wounds; in addition, it can adsorb bacteria.

Charcoal cloth may contain traces of elements used in the activationprocedure. As can be seen, the nature of the product has made it easy tointroduce other materials, such as bactericides, subsequently. Charcoalcloth post-impregnated with silver is also known, as a chemisorbent. Itwould nevertheless be desirable to extend the utility of charcoal cloth,e.g. in surgery, to take account of its inherent characteristics and tosupplement them with properties which are not disadvantaged in themanner described above or in general as the result of mere surfaceapplication, by post-impregnation, of a desired additive.

According to the present invention, an activated charcoal product has,in addition to any activating material, a metal uniformly dispersedtherein.

Metal elements of the compounds used to activate carbon in GB-A-1301101are Zn, Al, Ca, Mg, Fe (which all have halides with the common,apparently essential, Lewis acid characteristics), Pb, Co and Ba. Themetals used in the present invention are intended to provide the productwith additional, beneficial properties, e.g. catalytic or bactericidal.Suitable metals for use in the invention are those of the Group VIIIelements such as Fe or those of at. no. 76 to 78, e.g. Ir and Pt, andGroup Ib, e.g. Ag or Cu.

It would obviously save time, labour and cost if, say, charcoal clothcould be impregnated with, say, the bactericidal metal silver, by use ofa suitable silver compound at the same time as the activating compounds.For example, a procedural step is saved if the desired metal could beintroduced with the halide solution used, before the heating steps, inthe procedure of GB-A-1301101, rather than by impregnation after theactivated material has been obtained.

However, if a solution of a soluble silver salt such as silver nitrateis added to a solution of halides as described in GB-A-1301101,insoluble silver halide is precipitated out. The result is poor,nonuniform impregnation of the charcoal cloth, or even no impregnationwhatsoever, and the presence of potentially undesirable nitrate. Aconventional attempt to overcome this problem, e.g. by the addition ofammonia which dissolves and prevents precipitation of silver chloride byforming complexed ions, causes precipitation of the activating elementszinc and aluminium, as their hydroxides, from the activating solution.

According to a second aspect of the present invention, a process for thepreparation of an activated carbon product comprises treating a fibrouscarbohydrate material with a solution of one or more Lewis acid halidesof Zn, Al, Ca, Mg and Fe; a compound of a further metal element whosehalide is relatively insoluble with respect to the, or the mixture of,Lewis acid halides; ammonia; and a sequestering agent; and then drying,carbonising and activating the carbohydrate material. The drying,carbonising and activating may be conducted in conventional manner, e.g.by the procedures described in GB-A-1301101, the contents of which areincorporated herein by reference.

The process of the invention allows the preparation of a product of theinvention. The "further" metal element can be any of those describedabove as having desirable properties, supplementing those of theactivated product, and which have substantially insoluble halides. Addedmetal element can be uniformly dispersed in the product to give thedesired, e.g. catalytic or bactericidal activity. This can be achievedwithout the precipitation or other problems described above.

The sequestering agent is preferably a hydroxycarboxylic acid. Asuitable hydroxycarboxylic acid is tartaric acid. Citric acid ispresently most preferred.

The impregnating solution is suitably prepared by dissolving the Lewisacid or acids, e.g. a mixture of zinc and aluminium chlorides, in water,and then adding the sequestering agent, ammonia and a soluble salt ofthe desired metal. An ammoniacal silver halide solution could be used tosupply both silver and ammonia.

The quantities of the materials contained in the solution can bedetermined fairly readily by simple experiment. However as a guide, ifthe solution contains, by weight, 3% ammonium chloride, 3% zinc chlorideand 3% aluminium chloride hexahydrate, as mentioned above, it has beenfound that the addition of between 3% and 5% by weight citric acidprevents the formation of metal hydroxides when ammonium hydroxide isadded to the solution, though greater amounts may be used if desired,particularly if the chloride concentrations are increased. The 3% valuesfor chloride concentration are in fact optimum figures; as little as 2%could be used. 5% chloride might require about 7-8% citric acid.

Moreover, the amount of ammonium hydroxide required to suppress theformation of silver chloride (when the silver salt is added) isdependent on the amount of silver desired in the final impregnationsolution, and can be determined by experiment. if cloudiness is observedin the solution, further ammonia can be added to remove precipitation.

While the preferred halides are the chlorides, fluorides, bromides andiodides can be used in some cases (though evolution of HF duringcarbonisation is obviously disadvantageous). AgI and AgBr havesufficient solubility in ammonia to give solutions of the requiredconcentration for impregnation (e.g. less than 0.1% by weight Ag).

The above description can be generalised when it is desired to dispersea metal other than silver in the product. In determining the amount ofthe metal which is desired, in accordance with the precedingdescription, the yield of charcoal cloth given by any normal method ofmanufacture will be generally known or can be easily established. The,say, catalytic or bactericidal effect which is desired in the productcan be achieved at low levels. Thus, for example, the product willusually comprise at least 0.05 % or 0.1, and often at least 0.2, butneed not contain more than 5, 2 or even 1, and often no more than 0.5,by weight Ag or other desired metal. A product of the invention can beseen, by suitable microscopic examination, to have a uniformdistribution of very small particles of, say, silver or silver oxide,extending through the thickness of the product. It can be seen quiteclearly as distinct from a post-impregnated product, where relativelylarge agglomerations of the, say, silver of silver oxide are present,and at the surface of the product.

Other than the presence of the added material, a product of theinvention can have all the characteristics associated with activatedcarbon products. It may be produced in the usual way, e.g. from rayon.It may be a felt or a knitted or, typically, woven cloth. A cloth,typically from 0.2 to 1 mm thick, containing uniformly distributedsilver, can be advantageously used as a surgical dressing orchemisorbent.

The method and products of the inventions, and their utility, will nowbe illustrated.

EXAMPLE 1

To approximately 5 liters of tap water were added:

ammonium chloride: 225 g

zinc chloride: 225 g

aluminium chloride: 225 g

citric acid: 300 g

880 ammonia (fresh bottle): 900 cm³

An aqueous solution containing 15 g silver nitrate in c. 400 cm³distilled water was made up and acidified with nitric acid (20%; 5 cm³).(This addition is intended to prevent seed crystals of silver chlorideforming). This solution was kept stoppered and in a dark place.

The silver nitrate solution was added to the bulk liquid (stirred) inaliquots of about 25-50 cm³. White precipitation quickly disappeared.The addition of the final aliquot produced a persistent whiteprecipitation and a further addition of 880 ammonia (200 cm³) was made.The solution became clear again. Volume was adjusted to 7.5 dm³ to givean impregnation solution containing:

    ______________________________________                                        ammonium chloride                                                                           3%                                                              zinc chloride 3%                                                              aluminium chloride                                                                          3%                                                              citric acid   4%                                                              ammonia       ca 385 g (assuming 35% wt/vol)                                                (i.e. 1100 cm.sup.3)                                            silver nitrate                                                                              0.2%                                                            ______________________________________                                    

Three lengths of rayon cloth (25 cm×5 m) were dipped separately into ashallow trough containing the impregnation solution. Dipping time wasapproximately 2 seconds; rolls following on and being allowed to drainwith intermittent turning. The lengths were each rolled at 345 kPa andplant oven-dried (at 125° C.) by a single pass.

A silver analysis was made on a 3 g sample. The analysis was conductedby ashing a sample at 750° C., moistening the resultant ash withconcentrated nitric acid, and re-igniting the moistened sample toconstant weight, in order to ensure that all halides were expelled. Thefinal residue was boiled with 10 cm³ 8 M nitric acid and the entiresolution diluted and titrated directly against standardised potassiumthiocyanate solution using the Volhard procedure. The silver content was0.23% by weight.

EXAMPLE 2

Four pieces of rayon felt (25×46 cm) were separately dipped into ashallow trough containing the impregnation solution used in Example 1.Dipping time was approximately 5 seconds. The pieces were rolledconsecutively onto a 38 mm diameter tube and drained with intermittentturning. The pieces were ovendried without pressure by residence of someten minutes.

All samples were stored in polypropylene sheet and again in black pvc toreduce ultraviolet penetration pending charring of the samples. Allsamples were charred at 360° C. in carbon dioxide, followed byactivation in carbon dioxide at 950° C.

The silver content, by the analysis described in Example 1, was 0.40% byweight.

The cloth and felt products of Examples 1 and 2 have been demonstratedas active against Staphylococcus aureus (Oxford), Bacillus subtilis NCTC8236, E. coli DCO and its envelope mutant DC2, and Pseudomonasaeruginosa 799 and its envelope mutant 799/61. These results have beenobtained in broth and on agar at dilutions of up to 1:100 (at least forthe cloth of Example 1).

Products such as in the examples can be used in catalysis, e.g. thebreakdown of arsine and phosphine. The term "Activating" is often usedto describe only the heat-treatment of carbon products. Herein, the termalso describes those elements, solutions etc. conventionally used inactivating carbon.

I claim:
 1. A surgical dressing which consists essentially of anactivated charcoal cloth or felt having uniformly dispersed therein atleast one first activating metal element selected from Zn, Al, Ca, Mgand Fe, and Ag as a second metal element.
 2. A surgical dressingaccording to claim 1 in which the amount of silver is from 0.1 to 1percent by weight.